3/10+3/40+3/88+3/154+3/238+3/340
= (1/2 - 1/5) + (1/5 - 1/8) + (1/8 - 1/11) + (1/11 - 1/14) + (1/14 - 1/17) + (1/17 - 1/20)
= 1/2 - 1/20
= 9/20
3/10+3/40+3/88+3/154+3/238+3/340
= (1/2 - 1/5) + (1/5 - 1/8) + (1/8 - 1/11) + (1/11 - 1/14) + (1/14 - 1/17) + (1/17 - 1/20)
= 1/2 - 1/20
= 9/20
• 高。看看这个:1x2+2x3+...+19x20 -yma16- ♂ (0 bytes) () 07/16/2024 postreply 11:26:08
• 可用此式裂项:n(n+1)=[(n+1)^3-n^3-1]/3 -万斤油- ♂ (0 bytes) () 07/16/2024 postreply 20:49:05
• 这类的和有公式。 -yma16- ♂ (0 bytes) () 07/16/2024 postreply 22:09:11
• 比如 -yma16- ♂ (4398 bytes) () 07/16/2024 postreply 22:10:57